Method of producing a single crystal of or thoferrite and thin platelets thereof by means of the floating zone method

ABSTRACT

An orthoferrite single crystal is grown by the floating zone method with the growth direction of the crystal perpendicular to the easy axis of magnetic anisotropy by using a starting seed crystal whose easy axis is disposed perpendicular to the growth direction. The thus produced crystal is then cut into thin platelets in which the plane surfaces thereof are perpendicular to the easy axis of magnetic anisotropy.

United States Patent 11 1 Makino et al.

[ METHOD OF PRODUCING A SINGLE CRYSTAL OF ORTHOFERRITE AND THINPLATELETS THEREOF BY MEANS OF THE FLOATING ZONE METHOD [75] Inventors:Hiroshi Makino; Koichi Matsumi,

both of Tokyo, Japan [73] Assignee: Nippon Electric Company, Limited,

Tokyo, Japan [22] Filed: Sept. 16, 1971 [2]] Appl. No.: 181,035

[30] Foreign Application Priority Data Oct. 9, 1970 Japan 45-89112 [52]US. Cl. 23/305, 23/DIG. 1., 23/301 SP, 252/6257, 423/263, 423/594 [51]Int. Cl B01j 17/10 [58] Field of Search 23/305, 301 SP, DIG. 1, 23/51,300; 252/6257; 423/594, 263

[56] References Cited UNITED STATES PATENTS Barnes 23/301 tmj 3,801,290[45 Apr.'2,- 1974 3,009,788 Daimon 23/301 1 1 1961 3,272,591 9 1966Rudness.... 23 301 3,414,372 12/1968 Paulus 23 301 3,429,818 2/1969Benedetto 23/305 2,809,136 10/1957 Mortimer 23/301 OTHER PUBLICATIONSAkashi, et al., Prep. of Ferrite Single Crystals by New Floating ZoneTech., IEEE Trans. Mag., -Vol. Mag. 5, pp. 285-289, (9/69).

Primary Examiner-Wilbur L. Bascomb, .lr.

Assistant Examiner-R. T. Foster Attorney, Agent, or Firm-Sandoe, Hopgoodand Calimafde; Eugene J. Kalil ABSTRACT An orthoferrite single crystalis grown by the floating zone method with the growth direction of thecrystal perpendicular'to the easy axis of magnetic anisotropy by using astarting seed crystal whose easy axis is disposed perpendicular to thegrowth direction. The thus produced crystal isthen cut into thinplatelets in which the plane surfaces thereof are perpendicular to theeasy axis of magnetic anisotropy.

4 Claims, 5 Drawing Figures PATENTEDAPR 2:914 3.801.290

INVENTORS A l/P0567 MA/f/IVO BY A o/cw/ M47504 MWW METHOD OF PRODUCING ASINGLE CRYSTAL OF ORTHOFERRITE AND THIN PLATELETS THEREOF BY MEANS OFTHE FLOATING ZONE METHOD This invention relates to a method of producinga single crystal of orthoferrite by the floating zone method and thinplatelets thereof.

BACKGROUND OF THE INVENTION motions and moreover restrict the size ofthe effective area of the device.

. OBJECTS OF THE INVENTION It is therefore an object of this inventionto provide a method of producing an orthoferrite single'crystal fromwhich improved bubble domain devices can be producted. I

Another object is to provide a method of manufac-- turing wide-area thinplatelets of orthoferrite.

It is still another object to provide a method of manufacturing bubbledomain devices having a substantially wide or large area andsubstantially without defects.

SUMMARY OF THE INVENTION According to this invention, a method isprovided of producing an orthoferrite single crystal by floating zonemethod, said crystal having a uniaxial magnetic anisotropy, wherein theimprovement resides in the step of growing the crystal along an axisperpendicular to the' easy axis of magnetic anisotropy.

According to one embodiment of the invention, there is provided a methodof manufacturing thin platelets out of an orthoferrite single crystalproduced by floating zone method, the crystal having a uniaxial magneticanisotropy, wherein the improvement comprises the steps of growing thecrystal along an axis perpendicular to the easy axis of magnetization,the single crystal thus produced is cut into thin platelets havingparallel surfaces perpendicular to the easy axis of magnetization.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of theessential portion of an apparatus for growing a single crystal byfloating zone method, which is used in carrying out the method accordingto this invention;

FIG. 2 is a schematic perspective view of a single crystal oforthoferrite grown by a conventional method;

FIG. 3 is a like view of a single crystal of orthoferrite cut into thinplatelets according to a conventional method;

FIG. 4 isa schematic perspective view of a single crystal oforthoferrite grown in accordance with the invention; and

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. I, theessential portion of an apparatus for growing a single crystal byfloating zone method comprises a lower axle I having a lower chuck 2 forholding a seed 3 of crystal onwhich a singlecrystal 4 is to be grown.The apparatus further comprises an upper axle 5 having an upper chuck 6for supporting a polycrystalline rod 7 to be subjected to the floatingzone method. By axial movement of the axles l and 5, the seed 3 and therod 7 are brought into contact at a predetermined position where theheat from a heat source (not shown) is caused to melt the interfaceportions of the seed 3 and the rod 7. With the relative positionunchanged, the axles l and 5 are lowered in the direction shown byarrows D. The single crystal 4 grows on the seed 3 from-molten zone 8which moves upwards relative to the axles l and 5.

With the apparatus of this type, it is already known that a singlecrystal 4 can be grownalong a particular crystallographic axis when thataxis of the seed 3 is placed parallel to the direction D. It has nowbeen found that a so-grown single crystal 4 of'orthoferriteor asubstance, such as'an oxide, having a large ionization tendency has aspecific crystal habit and a characteristic shape. f v

' It should be noted here that'the floating zone 8 is subjected tolocalized heat to a temperature above the melting point. As a result,the single crystal 4 grown on the seed 3 passes through asteeptemperature gradient whereby thermal stresses are imposed on thecrystal. When the single crystal 4 has a habit of growing into agenerally rectangular parallelepiped configuration (an elongatedcrystal), the stress'is marked at the four corners transverse to thelongitudinal axis. More particularly, the four corners are characterizedby many dislocations which probably result from crystallographic slipdue to the imposed high thermalstresses,

Referring to FIGS. 2 and 3, an elongated single crystal 9 of theorthoferrite having the easy axis of magnetization in the direction ofthe 00l axis is grown along the 001 axis, that is, along-longitudinalaxis X-X Cutting the single crystal 9 perpendicular to the axis ofgrowth, thin platelets l0, l1, 12, etc., are obtained. When, forexample, the platelet 10 is etched by hot phosphoric acid afterpolishing, it is observed by a metallurgical microscope that the etchpits are abundant at the four corners l3, l4, l5 and 16 and scarce atthe other surface portion between the corners. The etch pits show thedislocations caused by high thermal stresses set upon the single crystal9 at the four comers l3, l4, l5 and 16 formed in accordance with thehabit.

Since the single crystal is imperfect because of the abundantdislocations at the four comers, the performance of the bubble domaindevices formed of thin platelets 10 is adversely affected where themagnetic domains are driven two-dimensionally within platelet 10. Thisis because the dislocations impede the movement of the magnetic domains.A bubble domain device is usually manufactured by cutting the fourcorners l3, l4, l5 and 16 away. However, it is necessary that the devicehave the largest or widest possible area and also that the magneticdomains should be movable throughout the area. Thus, the conventionalmethod of manufacturing bubble domain devices has certain inherentdisadvantages in that the serviceable area of the device is markedlyreduced.

Referring to FIGS. 4 and 5, an elongated single crystal'l7 oforthoferrite having the easy axis of magnetization in the direction ofthe l axis is grown with the 0l0 axis of the seed 3 held parallel to orcoaxial with the axis of the apparatus, the longitudinal axis YY of thecrystal being perpendicular to the 00] axis. in accordance with thehabit, a cross section 18, namely, the (010) surface taken perpendicularto the axis of growth has a generally rectangular shape having thelonger sides parallel to the 00l axis. The etch pits observed asmentioned with reference to FIG. 3 are abundant at the four corners 19,20, 21 and 22 of the rectangle. Even with this invention, it isimpossible to obviate the dislocations. It is, however, possible to cutthe single crystal 17 into a multiplicity of thin platelets, such as 23,24, 25 and 26. Thin platelets 24, 25, with a substantial decrease in orhaving no dislocations are quite useful in bubble domain devices andhave larger areas despite the fact that they are cut from a singlecrystal l7 having almost the same volume as the conventional singlecrystal 9 shown in FIGS. 2 and 3. It will be noted, however, that thearea of working faces of platelets 24 and 25 without the dislocationsare much larger than the area of working faces of platelets 10, 11 and12 o-fFIG; 3. I I

I EXAMPLE 1 A single crystal of yttrium orthoferrite (YFeO was grown onthe seed of the same material placed with the 0l0 axis parallel to theaxis of the apparatus for growing the single crystal by floating zonemethod. The single crystal had the dimensions shown in FIG. 4. With theinitial and the last grown portions of the crystal of a length of aboutl0 mm each cut away, the 30 mm long single crystal remaining is then cutwith a slicing machine into eight sheets of thin platelets with theplanes thereof being the (001) planes. Four of the platelets had noportions abundant with the dislocations. With each of the thin platelets30 mm long and 5.5 mm wide, 'it was possible to drive the bubble domains over the whole area without any impediment.

As a reference, a single crystal of the same material was grown alongthe 00I axis in accordance with the conventional method. The singlecrystal had the dimensions shown in FIG. 2. With the initial and thelast grown portions ofa length of mm each cut away, the 30 mm longsingle crystal remaining is then cut into thirty-eight sheets of thinplatelets having the (001) planes. Each thin platelet of the generallysquare shape of 6 mm by 6 mm had only an effective area of4 mm by 4mm asa bubble domain device on account of the four-corner portions in whichthe dislocations were abundant.

The-effective area attained in accordance with this invention is morethan ten times as wide or larger as is obtained with the conventionalmethod.

EXAMPLE 2 A single crystal of yttrium orthoferrite (YFeO was singlecrystal had the dimensions illustrated in FIG. 4 except the axesand 0l0are interchanged.

The crystal was cut with a slicing machine into eight sheets of thinplatelets in the manner shown in FIG. 5. Four of them were free from theportions in which the dislocations were abundant. With each of the thinplatelets 30 mm long and 5.5 mm wide, it was possible to drive thebubble domains over the whole area as was the case with Example l.

EXAMPLE 3 The same results as described in conjunction with Example 1were achieved for a single crystal of terbium orthoferrite (TbFeO exceptthe single crystal was grown on the seed of terbium orthoferrite.

EXAMPLE 4 The same results as described in conjunction with Example 2were attained for a single crystal of terbium orthoferrite (TbFeO exceptthe single crystal was grown on the seed of terbium orthoferrite.

EXAMPLE 5 wherein Co ions were substituted for a portion of Fe ions.

EXAMPLE 7 The same results as described in connection with Examples land 2 were achieved for a single crystal of a cobalt-titaniumsubstituted yttrium orthoferrite (YFe- Co,, Ti,, O wherein an equalnumber of Co ions and Ti ionswere substituted. for a portion of Fe ions.I

From the Examples described above,-this invention is apparentlyapplicable to single crystals of all kinds of orthoferrite, wherein eachhas'a uniaxialmagnetic anisotropy and is manufacturable by the floatingzone method. Furthermore, it is possible to manufacture those thinplatelets of a single crystal of orthoferrite which have (001) surfaces,by growing the single crystal along an axis, such as 1 10 axis, that isperpendicular to the 00l axis. it should be noted here that thenotations of the crystallographic axes and planes mentioned above arenot limitative. For example, thin platelets having (100) surfaces aremanufacturable in accordance with this invention from a single crystalof orthoferrite, such as samarium orthoferrite, wherein the easy axis ofmagnetic anisotropy is the l00 axis.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are consid- 6 growth andperpendicular to said easy axis of magneti'zation.

2. A method as claimed in claim 1 wherein said crystal cutting step iscarried out at that portion of the single crystal in which thedislocations are substantially reduced.

3. A method as claimed in claim 1 wherein the easy axis of magnetizationis the axis 00l and said para] lel surfaces are (001) planes.

4. A method as claimed in claim 1 wherein the easy axis of magnetizationis the axis l00 and said parallel surfaces are planes.

2. A method as claimed in claim 1 wherein said crystal cutting step iscarried out at that portion of the single crystal in which thedislocations are substantially reduced.
 3. A method as claimed in claim1 wherein the easy axis of magnetization is the axis <001> and saidparallel surfaces are (001) planes.
 4. A method as claimed in claim 1wherein the easy axis of magnetization is the axis <100> and saidparallel surfaces are (100) planes.